Fluorescent lamps are efficient light sources. Fluorescent lamps have a wide variety of domestic and industrial applications, including lighting rooms, work spaces and signs, for example. In general, fluorescent light fixtures comprise one or more fluorescent lamps, each lamp providing a separate light source. Fluorescent lamps can vary in size, with larger lamps generally drawing more power and providing more light.
Fluorescent lamps are a gas discharge type of light source. A typical prior art fluorescent lamp 10 is shown in FIG. 1, along with its ballast 12, its power supply 14 and its starter switch 20. Lamp 10 also contains a small amount of mercury (initially in a substantially liquid or amalgam form) and one or more inert gases, usually argon, which are under low pressure (e.g. a 1-5 torr). Ballast 12 conventionally comprises a ferromagnetic inductor 13. Fluorescent lamp 10 comprises a pair of electrodes 16, 18. Electrodes 16, 18 can act as anodes (positively charged) or cathodes (negatively charged). When electrodes 16, 18 act as cathodes, they can introduce electrons into the low pressure gas of lamp 10. The cathodes are typically heated to promote thermionic emission of electrons. For this reason, electrodes 16, 18 typically comprise filaments 16A, 18A which are coated with thermionic emission materials and which are capable of being heated to thermionic emission temperatures. Typical filaments 16A, 18A require heating power on the order of 0.5-5 Watts.
For lamp 10 to create light, there must be current flow or “arc” through lamp 10 (i.e. between electrodes 16, 18). Creating a current arc through lamp 10 typically involves providing a relatively large “ignition voltage” between electrodes 16, 18. The ignition voltage induces ionization of the inert gas in lamp 10 and initiates current flow between electrodes 16, 18. The required ignition voltage for a given lamp 10 depends on many factors. Typical commercial fluorescent lamps of the “hot cathode” type operate with an ignition voltage in a range between 150V-800V AC RMS. Preheating of filaments 16A, 18A tends to reduce the required ignition voltage. Typically, the ignition voltage is provided between electrodes 16, 18 by ballast 12, which works together with starter switch 20 as explained briefly below.
During preheating, starter switch 20 is closed and AC preheat current flows through inductive ballast 12, filament 16A, switch 20 and filament 18A. Typically, this preheat current is at the same frequency as that of the ignition signal and the operating signal, which may be 60 Hz, for example. The preheat current heats filaments 16A, 18A, resulting in the emission of electrons. The preheat current also induces a magnetic field in inductor 13 of ballast 12. During preheating, there may be some ionization of the gas in lamp 10; however, during preheating, the voltage across lamp 10 (i.e. between electrodes 16, 18) is not sufficient to create a current arc through the gas in lamp 10. Consequently, all current flows through starter switch 20 and no current flows through lamp 10.
When electrons are being emitted from filaments 16A, 18A in sufficient quantity and inductor 13 has been sufficiently charged, starter switch 20 is opened. When the current flow through switch 20 is cut off, the magnetic field induced in inductor 13 collapses, causing an inductive voltage spike. This inductive voltage spike provides the ignition voltage across lamp 10 (i.e. between electrodes 16, 18), which in turn ionizes the gas in lamp 10 and creates an arc of current that flows between electrodes 16, 18.
After an arc has been initiated, current now flows through lamp 10. Current flow is maintained through lamp 10 by electrons emitted from hot filaments 16A, 18A and by the ionized gas particles in lamp 10. When current starts to flow through lamp 10, filaments 16A, 18A start to cool down somewhat because current is no longer flowing through switch 20 and through filaments 16A, 18A. However, filaments 16A, 18A tend to stabilize at a slightly reduced temperature because current flow through lamp 10 tends to heat filaments 16A, 18A (when electrodes 16, 18 are acting as cathodes). Arc current flowing through electrodes 16, 18 tends to heat filaments 16A, 18A sufficiently to maintain the filaments 16A, 18A at an emitting temperature.
Heat generated by the arc discharge in lamp 10 provides energy to the mercury in lamp 10, increasing its vapor pressure. Collisions between charged particles and gaseous mercury atoms cause electrons in the gaseous mercury atoms to occupy higher energy states. When these mercury electrons return to their ground energy state, they release ultra-violet photons. Lamp 10 is typically coated with phosphors (not shown), which absorb the ultraviolet photons. Absorption of ultraviolet photons causes the electrons of the phosphor atoms to occupy higher energy states. When these phosphor electrons return to their ground energy state, they release photons in the visible spectrum.
While an arc is maintained through lamp 10, the resistance through lamp 10 (i.e. between electrode 16 and electrode 18) decreases. More specifically, the flow of electrons and ions though lamp 10 creates collisions with other atoms, liberating more ions and electrons and facilitating the flow of more current. Inductive ballast 12 helps prevent damage to filaments 16A, 18A and lamp 10 by limiting the total current through lamp 10. Since power supply 14 provides a known AC signal, the inductance of inductor 13 may be selected appropriately to limit the current through lamp 10 to a desired level.
A significant drawback of prior art ballasts is cathode degradation. As discussed above, filaments 16A, 18A are typically coated with thermionic emission materials to increase electron emission. Evaporation and/or ion bombardment can remove these materials from filaments 16A, 18A and may cause deposition of these materials on the glass walls of lamp 10 in a process referred to as “sputtering”. As thermionic emission material is sputtered onto the glass walls of lamp 10, the material can trap gas molecules contained in lamp 10, reducing the internal gas pressure within lamp 10. Sputtering is a significant cause of damage to, and failure of, fluorescent lights.
Sputtering is caused by evaporation of thermionic emission material from filaments 16A, 18A when filaments 16A, 18A are overheated, for example, by the preheating current and/or the operating current. It is desirable, during preheating and operation, to increase the temperature of filaments 16A, 18A to a level where electrons are thermionically emitted from filaments 16A, 18A, while preventing the temperature of filaments 16A, 18A from increasing to the point where thermionic emission material evaporates from filaments 16A, 18A.
Sputtering is also caused by ion bombardment when the voltage difference between a filament 16A, 18A and the gas which surrounds filament 16A, 18A is too high. Ion bombardment typically occurs when this voltage difference is on the order of 3.5V-4V or higher. Under such conditions, positive gaseous ions in lamp 10 may accelerate towards filaments 16A, 18A with velocities which can cause impact damage to filaments 16A, 18A. When the voltage difference between a filament 16A, 18A and the surrounding gas is less than 3.5V-4V, the positive ions typically do not accelerate to damaging velocities. Sputtering caused by ion bombardment is prevalent during preheating, when the number of electrons that have been emitted from filaments 16A, 18A is relatively low.
Lamp damage caused by sputtering reduces the useful life of flourescent lamps. Typical prior art ballasts provide up to 100,000 lamps starts, after which the damage to the lamp has become so significant, that the lamp is unusable. In addition, sputtering reduces the efficiency of fluorescent lamps. Typical prior art lamps are about 30% efficient (i.e. in terms of a ratio of power coming out in the form of light energy to electrical input power), but this efficiency drops with age as sputtering causes blackening of the lamp inner surfaces and also causes an increasing loss of internal gas pressure. Within a few years of operation, the efficiency of prior art lamps has typically reduced to approximately half of their original efficiency (˜15%).
Fluorescent lamps, known as “rapid start” lamps, incorporate the same basic principles as the lamps described above, except that rapid start ballasts are designed to provide heater current (to filaments) at all times. Other modern fluorescent lamps, known as “instant start” lamps, incorporate a ballast design which eliminates the preheating stage and ignites current flow through the lamp with exceptionally high voltage signals. The exceptionally high voltages associated with instant start lamps can cause additional damage to the filaments.
Some fluorescent lighting incorporates electronic ballasts which use inverters to transform the 60 Hz power line frequency to a higher frequency signal, typically in a range of 20 kHz-50 kHz. Fluorescent lights incorporating electronic ballasts also suffer from filament degradation due to sputtering.
It is generally desirable to provide economical methods and systems for operating fluorescent lights which reduce filament degradation due to sputtering.
Another drawback with prior art fluorescent lamps is that they are not conducive to wide range dimming which is desirable for energy conservation. Various efforts have been made to provide dimming ballasts in the prior art, but have had limited success because of general public disinterest and because of the high price of components for such dimming ballasts. It is desirable to provide economical systems and methods for starting and operating fluorescent lights which allow the light emitted from a fluorescent lamp to be efficiently dimmed over a relatively large controllable dimming range.